Evolution and Natural Selection Flashcards

Species Definition

• Defining a "species" isn't straightforward and involves three concepts that may not always align.

Biological Species Concept

• Defines species as organisms that can interbreed and produce fertile offspring.
• Different-looking organisms can belong to the same species if they produce viable offspring.
• Offspring from such interbreeding are often called "hybrids.”

Morphological Species Concept

• Defines species as organisms that look alike and share morphological traits.
• Identical-looking organisms may belong to different species if they cannot produce offspring.

Phylogenetic Species Concept

• Defines species as organisms that share identical gene sequences.
• Considered the most accurate in evolutionary terms, as it elucidates the degree of relatedness among organisms.

Macroevolution vs. Microevolution

• Evolutionary study is divided into two general areas.

Macroevolution

• The study of evolutionary changes at the level of anatomical features.

Microevolution

• The study of evolutionary changes at the genetic (molecular) level.

Common Misconceptions About Evolution

• Individuals do not evolve; populations evolve.
• Evolution occurs at the population level.
• The study of microevolution at the population level is termed population genetics.

The Modern Synthesis

• Population genetics studies how populations change genetically over time.
• It integrates Mendelian genetics with Darwin's theory of evolution by natural selection.
• This synthesis focuses on populations as units of evolution.

Gene Pools and Allele Frequencies

• A population is a localized group of individuals capable of interbreeding and producing fertile offspring.
• The gene pool is the total aggregate of genes in a population at any given time.
• It consists of all gene loci in all individuals of the population, including all alleles.

The Hardy-Weinberg Theorem

• Describes a population that is not evolving.
• States that allele and genotype frequencies in a population's gene pool remain constant from generation to generation, provided that only Mendelian inheritance and recombination are at work.
• Mendelian inheritance preserves genetic variation, while natural selection can remove variation.

Allele Frequencies and Hardy-Weinberg Equilibrium

• Allele frequencies are calculated for both males and females in a population.
• If p represents the dominant allele frequency and q represents the recessive allele frequency, then (p + q) = 1.0.
• Algebraic simplification leads to the equation p^2 + 2pq + q^2 = 1, where:
  • p^2 is the frequency of the homozygous dominant genotype.
  • 2pq is the frequency of the heterozygous genotype.
  • q^2 is the frequency of the homozygous recessive genotype.
• If allele frequencies are not undergoing selection, the total allele frequencies of p and q will equal 1 in subsequent generations, indicating Hardy-Weinberg equilibrium.

Hardy-Weinberg Equilibrium and Selection

• Example: Flower color controlled by alleles R (red) and r (white).
  • Generation 1: All plants are Rr (pink flowers).
  • Generation 2: RR, Rr, and rr genotypes appear in a 1:2:1 ratio due to random segregation of alleles.
• If no selection occurs, these allele frequencies remain the same across generations; this is modeled by the Hardy-Weinberg Theorem.
• If 500 plants are counted (1000 alleles), with 320 RR (red), 160 Rr (pink), and 20 rr (white):
  • Frequency of R allele = (320 "," 2 + 160 "," 1)/1000 = 0.8 (80%).
  • Frequency of r allele = (160 "," 1 + 20 "," 2)/1000 = 0.2 (20%).
• If red flowers are selectively eaten:
  • New allele frequencies are calculated based on the remaining pink and white plants.
  • If the calculated frequencies deviate from the Hardy-Weinberg equilibrium, it indicates that selection has occurred.
• For example, if only 180 pink and white plants remain (360 alleles) after deer eat the red flowers:
  • R(p) = 180/360 = 0.5
  • r(q) = 20/360 = 0.06
  • Then, p^2 + 2pq + q^2 = 0 + 2(0.5)(0.06) + 0.06^2 = 0.096.
  • The population is no longer in Hardy-Weinberg equilibrium, suggesting selection.

Rules for Hardy-Weinberg Equilibrium

• If any of these rules are broken, allele frequencies will change, and evolution occurs:
  • Extremely large population size: Smaller populations increase the chance of genetic drift, reducing genetic diversity.
    • If broken: Genetic drift.
  • No Gene Flow: Transfer of alleles from inter-population matings can alter allele frequencies.
    • If broken: Introduction of new alleles.
  • No Mutations: Changes in chromosome structure can reproductively isolate members, preventing viable offspring.
    • If broken: Postzygotic isolation.
  • Random Mating: Preferential mating alters gamete mixing.
    • If broken: Sexual selection.
  • No Natural Selection: Selective pressure determines differential reproductive success, altering allele frequencies.

Genetic Drift and Bottleneck Effect

• Genetic drift changes allele frequencies over time without selection.
• A genetic bottleneck is similar but results from massive mortality in the mating population.

Selection Types

• Selection can change allele frequencies over time; common types exist.
• Sickle Cell Anemia, Stabilizing Selection, and Human adaptation to the Malaria parasite are examples.

Natural Selection

• Natural selection results from:
  • Success in reproduction.
  • Accumulation of habitat-specific traits in a population.
• It adapts a population to its environment.
• When the environment changes, natural selection also changes.

Reproductive Barriers and Speciation

• Reproductive barriers restrict gene flow and can cause biological speciation.

Prezygotic Barriers

• Gametic Isolation

Postzygotic Barriers

• Reduced Hybrid Viability
• Reduced Hybrid Fertility
• Hybrid Breakdown

Evolution and Life's Diversity

• Modern living species are descendants of older, often extinct species (99.1% of species that have existed are extinct).
• All life shares degrees of relatedness; differences and similarities among species can be explained by descent with modification.
• Descent with modification results from adaptation to the natural environment due to natural selection.
• Adaptation and natural selection are key components of evolution.

Evolutionary Theory

• Unifies the broad field of Biology.
• An idea becomes a scientific theory when:
  • It can explain natural phenomena under different testing conditions.
  • It can guide further levels of questioning.

Resistance to Evolution

• The Origin of Species challenged deeply rooted Western culture and a centuries-old worldview.

How Evolution Theory Evolved

• Key figures and events that contributed to the development of evolutionary theory:
  • Linnaeus (classification)
  • Hutton (gradual geologic change)
  • Lamarck (species can change)
  • Malthus (population limits)
  • Cuvier (fossils, extinction)
  • Lyell (modern geology)
  • Darwin (evolution, natural selection)
  • Mendel (inheritance)
  • Wallace (evolution, natural selection)

Geology and Evidence Against Creationism

• Geology provided evidence against creationism by suggesting Earth was much older than 6,000 years, evidenced by formations like the White Cliffs of Dover and the Grand Canyon.
• Fossils indicated that many ancient organisms had gone extinct.

Darwin's Contribution

• Darwin's major points in Origin of Species:
  • Modern organisms are descendants of ancient organisms.
  • Natural selection results in evolution, defined as change over time, due to adaptation to environments.
  • Origin of Species took forty years to write, based on meticulous analysis of overwhelming evidence.

Darwin's Voyage on the HMS Beagle

• Darwin was a naturalist on the HMS Beagle, tasked with collecting and cataloging biodiversity in South America.

Darwin's Observations in the Galapagos

• Darwin noted that Galapagos species were similar but not identical to those in mainland Ecuador.
• The Galapagos Islands, formed by volcanic activity, provided new habitats for colonization.

Galapagos Marine Iguana

• Specialized adaptations of Galapagos marine iguanas:
  • Jaws and skull for scraping algae.
  • Limbs and tail for swimming.
  • Black coloration for camouflage and heat absorption.
  • Fat content for thermoregulation.
• Comparison with arboreal iguana (ancestral state):
  • Jaws and skull for accessing fruit and hunting insects.
  • Limbs and tail for climbing trees.
  • Coloration for camouflage in forests.
  • Fat content for quick energy reserves.

Galapagos Finches

• Descent with modification from ancestral species Geospiza fortis on the mainland.
• Examples of finch species and their adaptations:
  • Cactus eater (Geospiza scandens): long, sharp beak.
  • Seed eater (Geospiza magnirostris): large beak.
  • Insect eater (Certhidea olivacea): narrow, pointed beak.

Adaptive Radiation in Galapagos Finches

• When an ancestral species colonizes a new habitat, adaptive radiation can occur.
• Examples: medium tree finch, small tree finch, vegetarian finch, mangrove finch, woodpecker finch, cactus finch, sharp-beaked ground finch, warbler finch
• Each finch species adapted to different food sources and habitats.

Artificial Selection

• Human selective breeding has produced descent with modification (artificial selection).
• Example: Six varieties of the wild mustard plant through selective breeding.
  • Cabbage
  • Brussels sprouts
  • Kale
  • Kohlrabi
  • Cauliflower

Natural Selection and Adaptation

• Mantid species from different continents (flower mantid in Malaysia, stick mantid in Africa) share a common ancestor.
• Natural selection results in vastly different coloration patterns.
• Populations adapt to their habitats, blending in with vegetation to obscure themselves from predators and prey.

Convergent Evolution

• Similar selective pressures cause similar adaptive traits in different lineages (convergent evolution).
• Example: Sugar glider (Australia) and flying squirrel (North America).

Homologous Characters

• Characters descended with modification from a common ancestor.
• Example: Vertebrate appendages of the pectoral girdle (human, cat, whale, bat).
  • humerus
  • ulna
  • radius
  • carpals
  • phalanges

Vestigial Appendages

• The tails of human embryos.
• The hind limbs of ancient whales.

Darwin's Main Ideas

• Evolution explains life's unity and diversity.
• Natural selection causes adaptive evolution.

Descent with Modification

• Summarizes Darwin's perception of the unity of life.
• All organisms are related through descent from a common ancestor.
• The history of life is like a tree with branches representing life's diversity.

Phylogeny and Evolutionary History of the Elephant Family

• Evolutionary relationships among different elephant species and their ancestors.
• Examples: Hyracoidea, Sirenia, Moeritherium, Barytherium, Deinotherium, Mammut, Platybelodon, Stegodon, Mammuthus, Elephas maximus, Loxodonta africana, Loxodonta cyclotis

Natural Selection Experiment

• Experimental transplant of guppies between pools with different predators (killifish and pike-cichlids).
• Guppies in killifish pools are larger at sexual maturity.
• Guppies in pike-cichlid pools are smaller at sexual maturity.

Results of Guppy Transplant Experiment

• After 11 years, transplanted guppies shifted from being small to being large.
• This demonstrates natural selection at work.

Ernst Mayer's Dissection of Darwin's Logic

• Based on five observations:
  • Observation 1: Populations increase exponentially if all individuals reproduce successfully.
  • Observation 2: Populations tend to remain stable in size.
  • Observation 3: Resources are limited.
  • Inference 1: Struggle for existence leads to only a fraction of offspring surviving.
  • Observation 4: Members of a population vary extensively.
  • Observation 5: Most variation is heritable.
  • Inference 2: Survival depends on inherited traits; individuals with favorable traits have higher fitness.
  • Inference 3: Unequal ability to survive and reproduce leads to gradual change in a population with favorable characteristics accumulating.

Summary of Natural Selection

• Natural selection is differential success in reproduction.
• It results from environmental changes that favor adaptive traits and natural variation.
• Over time, natural selection increases the adaptation of organisms to their environment.
• If the environment changes, natural selection may result in adaptation to new conditions.